Gadolinium oxysulfide (GOS) phosphors are used in x-ray screens as x-ray image intensifiers. Examples of these phosphors are given in U.S. Pat. Nos. 3,725,704 and 4,536,436 which are incorporated herein by reference. Phosphor brightness is a key parameter affecting the utility of x-ray intensifying screens and is typically expressed in terms of x-ray optical fluorescence (XOF) brightness (also referred to as plaque brightness) or screen brightness. Screen brightness is determined by forming an x-ray screen containing the phosphor and a binder and then using the screen to create an x-ray film exposure. Because it measures the brightness of the phosphor in situ, screen brightness is a more accurate means of determining the ultimate performance of the phosphor.
In order to produce high quality x-ray screens, it is necessary for the phosphor to be deagglomerated to create the proper particle packing so that maximum screen brightness may be realized. Unfortunately, GOS phosphors such as terbium-activated gadolinium oxysulfide (GOS:Tb) are highly susceptible to brightness loss brought about by the techniques used for deagglomeration. Since commonly used deagglomeration processes involve some type of particle-to-particle or particle to media impact, the deagglomeration processing step induces surface and/or bulk damage in the GOS phosphor particles and causes an increase in the number of fine particles (less than about 1 micron) which have intrinsically lower brightness. For example, after ball milling, microscopic examination of the phosphor particles reveals that some fraction of the primary particles have been broken into two or more pieces. More vigorous ball milling results in more broken particles and lower brightness. Other conventional milling processes such as vibratory milling or attritor milling are thought to be even more damaging than ball milling. And, to a lesser extent, processing steps which involve particle-to-particle impact such as sieving may also reduce brightness.
It would therefore be advantageous to have a finishing process that ameliorates the damage caused by deagglomeration.